The metal-oxide-semiconductor field-effect transistor (MOSFET) is a type of field-effect transistor (FET). It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage is used for amplifying or switching electronic signals.
The MOSFET is by far the most common transistor in digital circuits, as hundreds of thousands or millions of them may be included in a memory chip or microprocessor. Since MOSFETs can be made with either p-type or n-type semiconductors, complementary pairs of MOS transistors can be used to make switching circuits with very low power consumption, in the form of CMOS logic.
The ability to modify the threshold voltage of a given MOSFET allows for greater flexibility in design. Threshold voltage (Vt) and/or work function can be tuned by depositing a dielectric cap layer on the MOSFET, followed by a high-temperature anneal process. Typically, aluminum is used for p-MOS shifts and lanthanum is used for n-MOS shifts.
Unfortunately, each of these materials has limitations. Lanthanum typically requires a very high-temperature anneal (e.g. >900° C.) which can make integration into process flows difficult, especially with replacement metal gates. Aluminum typically fails to provide a sufficient shift.
Therefore, there is a need in the art for methods and materials for modifying the threshold voltage of MOSFETs.